1. Technical Field
This disclosure generally relates to the field of radio frequency identification (RFID), and more particularly to reading RFID transponders.
2. Description of the Related Art
In general, an RFID system is a system of automatic identification of objects using information stored in and retrieved from RFID tags, also known as RFID transponders. The use of RFID to identify and track objects remotely and automatically has been steadily increasing in the past few decades. In enterprises, for example, RFID is used as a tool in supply chain management to improve the efficiency of inventory tracking and management. RFID is also widely employed around the world on roads and highways to collect toll payment in one application, and to identify travelers at borders in another application, such as the NEXUS frequent traveler program between the U.S. and Canada.
An RFID system typically includes RFID transponders and at least one RFID reader, also known as an RFID interrogator. Most RFID transponders contain two basic components. The first component is an antenna for receiving and transmitting RF signals. The second component is an integrated circuit (IC) that may be configured to store information, process information, modulate and demodulate RF signals. There are typically three types of RFID transponders: passive, active, and semi-passive. Passive transponders contain no internal power source and thus rely solely on the RFID reader as the power source. The active and semi-passive transponders have an internal power source, although in active transponders the internal power source is used both to power the IC and to broadcast a response signal to an RFID reader while in semi-passive transponders it is used to power the IC but not for broadcasting the response signal. In operation, an RFID reader transmits RF signals as queries and an RFID transponder receiving the queries generates and transmits a response signal, typically including the identification of an object which the responding RFID transponder is affixed to or otherwise associated with. The response signal is received by the RFID reader and is decoded for further processing.
As much as RFID is used in a wide variety of applications ranging from product tracking and inventory management to toll payments, there nevertheless are limitations associated with the use of an RFID system. For instance, at present time, there is a speed limit of about 15 miles per hour (mph) for reading transponders on moving objects. Beyond that speed, an RFID reader will likely fail to read the moving transponder or have significantly increased number of decode failures due to drooping in the amplitude of the received signal and inversion of the phase of the signal as a result of the speed. This means, for instance, the driver having a NEXUS card with an embedded RFID transponder or an RFID transponder in the form of a windshield sticker would have to slow down in order for the transponder to be read when crossing a border or a toll booth utilizing RFID technology. The fact that each driver has to slow down to a speed of lower than 15 mph can cause congestion on major roadways. Similarly, in other applications, the limit on the relative speed between an RFID transponder and an RFID reader likely imposes a problem when efficiency or time is of the essence.